Solid state light sources may be used to provide colored (e.g., non-white) or white light (e.g., perceived as being white or near-white). A solid state lighting device may include, for example, at least one organic or inorganic light emitting diode (“LED”) or a laser. White solid state emitters have been investigated as potential replacements for white incandescent or fluorescent lamps due to reasons including substantially increased efficiency and longevity. Longevity of solid state emitters is of particular benefit in environments where access is difficult and/or where change-out costs are extremely high. Solid state light sources provide potential for very high efficiency relative to conventional incandescent or fluorescent sources, but present challenges in simultaneously achieving good efficacy, good color reproduction, and color stability (e.g., with respect to variations in operating temperature).
Color reproduction is commonly measured using Color Rendering Index (CRI) or average Color Rendering Index (CRI Ra). In the calculation of CRI, the color appearance of 14 reflective samples is simulated when illuminated by a reference illuminant and the test source, and a difference in color appearance for each sample between the test and reference illumination is computed. CRI therefore provides a relative measure of the shift in surface color and brightness of an object when lit by a particular lamp. The general color rendering index CRI Ra is a modified average utilizing the first eight indices, all of which have low to moderate chromatic saturation. The CRI Ra equals 100 (a perfect score) if the color coordinates and relative brightness of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the reference radiator. Daylight has a high CRI (Ra of approximately 100), with incandescent bulbs also being relatively close (Ra greater than 95), and fluorescent lighting being less accurate (typical Ra of 70-80) for general illumination use where the colors of objects are not important. For some general interior illumination, a CRI Ra>80 is acceptable. CRI Ra>85, and more preferably, CRI Ra>90, provides greater color quality.
Aspects relating to the inventive subject matter disclosed herein may be better understood with reference to the 1931 CIE (Commission International de I'Eclairage) Chromaticity Diagram, which is well-known and readily available to those of ordinary skill in the art. The 1931 CIE Chromaticity Diagram maps out the human color perception in terms of two CIE parameters x and y. The spectral colors are distributed around the edge of the outlined space, which includes all of the hues perceived by the human eye. The boundary line represents maximum saturation for the spectral colors. The chromaticity coordinates (i.e., color points) that lie along the blackbody locus obey Planck's equation: E(λ)=A λ−5/(eB/T−1) where E is the emission intensity, λ is the emission wavelength, T the color temperature of the blackbody, and A and B are constants. Color coordinates that lie on or near the blackbody locus yield pleasing white light to a human observer. The 1931 CIE Diagram includes temperature listings along the blackbody locus (embodying a curved line emanating from the right corner). These temperature listings show the color path of a blackbody radiator that is caused to increase to such temperatures. As a heated object becomes incandescent, it first glows reddish, then yellowish, then white, and finally bluish. This occurs because the wavelength associated with the peak radiation of the blackbody radiator becomes progressively shorter with increased temperature, consistent with the Wien Displacement Law. Illuminants which produce light that is on or near the blackbody locus can thus be described in terms of their color temperature.
The term “white light” or “whiteness” does not clearly cover the full range of colors along the BBL since it is apparent that a candle flame and other incandescent sources appear yellowish, i.e., not completely white. Accordingly, the color of illumination may be better defined in terms of correlated color temperature (CCT) and in terms of its proximity to the BBL. The pleasantness and quality of white illumination decreases rapidly if the chromaticity point of the illumination source deviates from the BBL by a distance of greater than 0.01 in the x, y chromaticity system. This corresponds to the distance of about four MacAdam ellipses, a standard employed by the lighting industry. A lighting device emitting light having color coordinates that are within four MacAdam step ellipses of the BBL and that has a CRI Ra>80 is generally acceptable as a white light for illumination purposes. A lighting device emitting light having color coordinates within seven or eight MacAdam ellipses of the BBL and that has a CRI Ra>70 is used as the minimum standards for many other white lighting devices including compact fluorescent and solid state lighting devices.
General illumination generally has a color temperature between 2,000 K and 10,000 K, with the majority of lighting devices for general illumination being between 2,700 K and 6,500 K. The white area proximate to (i.e., within approximately eight MacAdam ellipses of) of the BBL and between 2,500 K and 10,000 K, is shown in FIG. 13 (based on the 1931 CIE diagram).
Luminous efficacy is a measure of how well a light source produces visible light, and represents the ratio of luminous flux to power (with the power being either radiant flux or total power consumed by a source, depending on context). Wavelengths of light outside of the visible spectrum are not useful for illumination because they cannot be seen by the human eye. Moreover, the human eye exhibits greater response to some wavelengths of light than to others, even within the visible spectrum. Response of the human eye to light also varies with respect to the level of intensity of light.
At the back of the eye, the retina contains millions of light receptors that convert light into electrified signals that are sent to vision centers of the brain. The retina contains two major categories of photoreceptors called cones and rods because of their geometric shapes. The very central part of the retina, called the fovea, contains only cones. The rest of the retina is populated with both rods and cones, with the number of rods exceeding the number of cones by a ratio of about 10 to 1.
Photopic vision is the vision of the eye under well-lit conditions. In humans and many other animals, photopic vision is mediated by cone cells, and allows color perception and significantly higher visual acuity than available with scotopic vision. The human eye uses three types of cones (with biological pigments having maximum absorption values at wavelengths of about 420 nm (blue), 534 nm (bluish-green), and 564 nm (yellowish green)) to sense light in three bands of color, providing maximum efficacy of about 683 lm/W at a wavelength of 555 nm (green). Scotopic vision is the vision of the eye under low light conditions. In the human eye, cone cells are nonfunctional in low light; as a result, scotopic vision is produced exclusively through rod cells, which are most sensitive to wavelengths of light around 498 nm (green-blue) and are insensitive to wavelengths longer than about 640 nm (red). For young eyes, scotopic vision may peak around 507 nm, with a sensitivity equivalent to about 1700 lm/W. Scotopic vision results in poor color discrimination. Scotopic vision occurs at luminance levels of 10−2 to 10−6 cd/m2; photopic vision occurs at luminance levels of 1 to 106 cd/m2 (normal light); and mesopic vision occurs in intermediate lighting conditions (luminance levels of 10−2 to 1 cd/m2) m, with mesoptic vision effectively being a combination of scotopic and photopic vision but yielding inaccurate visual acuity and color discrimination.
FIG. 14 illustrates scotopic and photopic luminosity functions, with the leftmost curve embodying a scotopic luminosity function (as adopted by the Commission Internationale de I'Éclairage (CIE) in 1951), and with the rightmost curve embodying photopic luminosity functions (wherein the solid line represents the CIE 1931 standard, the dashed curve represents the Judd-Vos 1978 modified data, and the dotted curve represents the Sharpe, Stockman, Jagla & Jägle 2005 data). The CIE 1931 photopic luminosity function also forms the central color matching function in the CIE 1931 color space. As shown in FIG. 1, the scotopic curve exhibits significant response above 420 nm, a peak at 507 nm, and very little response above 600 nm, whereas the photopic curve very limited response below 450 nm, a peak at 555 nm, and significant response above 650 nm before declining to zero response around 700 nm. For everyday light levels, the photopic luminosity function best approximates the response of the human eye; however, for low light levels, the response of the human eye changes, and the scotopic luminosity function applies. This difference in the scotopic and photopic luminous efficacy of the source results from the shift of eye sensitivity function peaking at 555 nm (under photopic lighting conditions) to 507 nm (under scotopic lighting conditions), also known as the Purkinje shift.
Historically, lighting manufacturers have utilized light meters to determine a lamps lumen output that are calibrated by examining the eye's sensitivity to only cone activated vision in the very central part of the retina, the fovea, while ignoring the effect of rod activated vision. As a result, traditional lighting practice accepted a single sensitivity function based on the assumption that the more light sensitive rods only functioned at very dim light levels. More recent studies have demonstrated that rod photoreceptors are active not only in dim light but also at typical interior light levels as well.
Since rods are more sensitive than cones to bluish-white light sources characteristic of higher correlated color temperature (CCT) values, light sources with higher S/P ratios may potentially provide equivalent levels of perceived brightness and visual acuity at lower output power level. This explains why environments lit by warm white (3000K) and even cool white (4100K) lamps (e.g., fluorescent lights) appear less bright than the same environment lit by lamps of a higher color temperatures, such as 5000K or above. Light with high S/P ratios, which provides higher correlated color temperature (CCT), yields relatively smaller pupils at a given photopic light level, so that object light rays are collected more at the central region of the eye, which may result in improved optical vision. Since an environment may be illuminated with lamps having higher S/P ratios at lower power levels to providing perceived brightness levels equivalent to those attainable with lamps having lower S/P ratios at higher power levels, use of lamps having higher S/P ratios may provide basis for saving energy.
Despite the potential for saving energy using lamps with higher S/P ratios, it is not straightforward to make high S/P ratio light sources with sufficient levels of color rendering (i.e., to enable color differentiation). There exists a trade-off between S/P ratio and color rendering index (CRI). This trend makes it challenging to provide a lamp capable of high S/P ratio and high CRI.
In terms of S/P ratio and CRI, the performance of current lighting technologies is typically limited. For example, incandescent light bulbs have a poor (low) S/P ratio of 1.41 despite a perfect CRI of 100. Among solid state light sources, traditional LED light sources including blue LEDs arranged to stimulate emissions of (yellow) yttrium aluminum garnet phosphors exhibit S/P ratios typically ranging from approximately 1.68 to approximately 2.38. Integration of different phosphors might improve S/P ratio or CRI, but such phosphors may also decrease luminous efficacy. A solid state lighting device including a blue LED combined with semiconductor nanocrystal quantum dots arranged to output cyan, green, yellow, and red emissions to provide higher S/P ratio values (e.g., exceeding 2.50) is disclosed by Nizamoglu, S., et al., “High scotopic/photopic ratio white light-emitting diodes integrated with semiconductor nanophosphors of colloidal quantum dots,” Optics Letters (May 15, 2011) Vol. 36, No. 10, pp. 1893-1895.
The art continues to seek improved lighting devices that address one or more limitations inherent to conventional devices.